1. Field of the Invention
The present invention relates to an image input device connected to a personal computer or a portable terminal, or an image input device used with a facsimile machine or digital copier.
2. Description of Related Art
Image sensors used for image input devices to read images of source documents are generally classified into a reduction projection type, a contact type and an absolute contact type. Since a device of the reduction projection type projects an image of a source document onto a CCD through a lens, the device tends to be large and therefore is not suitable for portable use. On the other hand, devices of the contact type and of the absolute contact type are thinner than that of the reduction projection type and also superior in operability.
In recent years, a slimmer, weight-reduced image input device for portable use which has higher resolution and is capable of reading images at higher speed has been sought after. In view of thickness and weight reduction and of high-speed reading, also used is a two-dimensional image sensor which does not need a mechanical part.
FIG. 6 is a sectional view illustrating the structure of a conventional image sensor of the absolute contact type. A glass substrate 101 is coated with a silicon oxide film 102 and a metal electrode 103 is formed thereon. Then a semiconductor film 104 having photoelectric effect is formed, an electrically conductive transparent film 106 is formed on a portion which serves as a photoelectric converter. An insulation transparent film 105 and a metal electrode 107 are deposited thereon in this order. Further a transparent layer 110 which also serves as an adhesion layer is applied thereon, and a thin glass plate 111 is adhered thereto. The metal electrode 103 is provided with a light let-in window 108 for letting light through in correspondence to the photoelectric converter. In this image sensor, light emitted by a light source 113 passes through the light let-in window 108 and illuminates a document 112. Light reflected according to contrast on the document is incident on the semiconductor layer 104 that serves as the photoelectric converter, and thus the image of the document 112 is read.
However, with the conventional image input device by use of an image sensor as described above, a first problem is that a sufficient S/N ratio cannot be obtained. In other words, if the area of the light let-in window 108 is increased with the view of increasing the amount of light illuminating the document 112 with retaining the resolution, the area of the photoelectric conversion means decreases. On the other hand, if the area of the photoelectric conversion means is increased, the amount of illuminating light decreases. Therefore, it is impossible to raise the resolution.
Further, since a boundary is formed between the transparent layer 110 and the thin glass plate 111 which have different refractive indexes, the illumination light is partly reflected at this boundary and becomes stray light. As a result, contrast declines. This is a second problem.
Furthermore, since the above-described image sensor does not have a lens, the photoelectric converter also receives reflected light from an adjacent pixel. As a result, the resolution declines. The larger the resolution is intended to be, the more remarkable the adverse effect thereof becomes. This is a third problem.
The first and third problems especially turn out to be more serious in the case of the two-dimensional image sensor. The reason is that, in the case of a one-dimensional image sensor, there is room in the direction perpendicular to a sensor array, i.e., in the direction perpendicular to a paper face in FIG. 6. Therefore, even if the width of the light let-in window 108 is reduced, thedepthdimensioncanbeenlarged. Ontheotherhand, in the case of the two-dimensional image sensor, it is impossible to enlarge the depth dimension and it is extremely difficult to keep a passageway for the illumination light. As for the influence of light from adjacent pixels, in the case of the one-dimensional image sensor, light from adjacent pixels on both sides, at most, has influence. In the case of the two-dimensional image sensor, however, light from all surrounding pixels has influence.
With such a structure that, with respect to the surface on which the photoelectric conversion means is disposed, the source document is placed thereabove and reflects illumination light incident from below the surface to make the reflected light incident onto the photoelectric converter, it is difficult to integrate the image sensor with a display device. This is another problem.
As other related art to the present invention, Japanese Unexamined Patent Publication (Kokai) No. Sho 63(1988)-214058 discloses an image sensor of the contact type wherein light from a light source is introduced by a transparent substrate, the optical path of the light is bent by a reflection plan to illuminate a document surface, light reflected on the document surface is collected by microlenses and an image of the document is formed on photoreceiver elements.
Japanese Unexamined Patent Publication (Kokai) No. Hei 5(1993)-347396 discloses an image reading device wherein a photoelectric converting layer is provided with a light let-in window to pass illumination light through.
Japanese Unexamined Patent Publication (Kokai) No. Hei 8(1996)-191371 discloses an image sensor wherein light from a light source is introduced by a light guiding layer, the light is transmitted by repeating the scattering and reflection by a light scattering layer and a light reflecting layer to illuminate a document surface, and light reflected from the document surface is directed to a photoreceiver element by a light passing section.
U.S. Pat. No. 5,430,462 discloses a technique of passing light through a device composed of a liquid crystal layer and a photo-conductive layer which are laminated, from the photo-conductive layer side to illuminate a document surface.